CONTRIBUTIONS TO THE CHEMISTRY OF SILICON-SULPHUR COMPOUNDS 62. THE ALCOHOLYSIS AND PHENOLYSIS OF ISOBUTYL(ISOPROPOXY)SILANETHIOLS ⁱBu_n(ⁱPrO)_3-nSiSH CATALYZED WITH SUBSTITUTED PYRIDINES

Abstract

The reaction rates of alcoholysis and phenolysis of isosteric silanethiols ⁱ Bu _n ( ⁱ PrO)_3-nSiSH n = 0 ÷ 3 catalyzed by pyridine and its moderately basic derivatives in acetonitrile solutions were measured under pseudo-first order conditions. The reactions are first order in respect to pyridine, to alcohol and to thiol. A positive Hammett correlation with p value of 0.616 was obtained in the case of phenolysis of ( ⁱ PrO)₃SiSH and with p value of 0.881 in the case of ⁱ Bu₃SiSH. A mechanism of general base catalysis was postulated. The spontaneous alcoholysis seems also to proceed according to general base catalysis. The catalytic rate constants k _c for methanolysis catalyzed by pyridine fall in sequence: ( ⁱ PrO)₃SiSH > ⁱ Bu( ⁱ PrO)₂SiSH ≤ ⁱ Bu₂( ⁱ PrO)SiSH > ⁱ Bu₃SiSH.

The rate constants for spontaneous methanolysis follow the sequence: ⁱ Bu( ⁱ PrO)₂SiSH > ⁱ Bu₂( ⁱ PrO)SiSH > ⁱ Bu₃SiSH ≤ ( ⁱ PrO)₃SiSH

The anomeric effect operating on silicon center in the ground state and (or) in the transition state (steroelectronic catalysis) seem to be responsible for these reactivity trends.

Key Words:

• Silicium
• silanethiols
• alcoholysis
• catalysis